The fight against atmospheric pollution has resulted, in many countries, in the passing of laws which require automobiles with internal combustion engines to employ various ancillary devices for the purpose of reducing as far as possible, the emission of noxious gases. Such devices do not always have a beneficial effect on the fuel consumption of the engine, but fuel economy is very important and is sought by most automobile manufacturers. In general the twin objects of reducing the emission of pollutants and increasing fuel economy (by reducing fuel consumption) are attained in the first place by effecting after burning of the exhaust gases by mixing into them further combustion air for completing, as far as possible, the combustion of the carbon monoxide and the unburnt hydrocarbons, in the second place by the addition to the incoming fuel/air mixture in the induction side of the engine a small proportion of the hot exhaust gases in order to reduce the formation of nitrogen oxides, and in the third place by means of the suitable metering of the fuel/air mixture in such a way that the engine always functions with optimal carburation.
However, internal combustion engines frequently experience transient operating conditions in which the above mentioned measures, such as for example the after burning of exhaust gases, can be damaging to the engine, especially its exhaust system inasmuch as it produces local overheating. Such conditions occur when the engine is hot, for example upon hard acceleration with the throttle wide open and on overrun when the throttle is closed; when the engine is cold, particularly immediately upon starting, such conditions can also arise. In all these cases, in fact, the engine is supplied with a very rich mixture the result of which is an increase in the unburnt hydrocarbons in the exhaust gases and consequently, upon afterburning, very high temperatures which can be damaging to the exhaust system.